1. Field of the Invention
The present invention relates generally to power conversion and in particular to power supplies. Still more particularly, the present invention relates to a switching controller for operating a flyback power converter in a critically continuous conduction mode to achieve high power factor and a method of operation thereof.
2. Description of the Related Art
Regulated DC power supplies are employed in various analog and digital electronic systems. The power supplies are typically designed to produce a regulated output, i.e., the output voltage is maintained within a specific range, with electrical isolation between the input and output. Additionally, power supplies may be designed to provide multiple outputs, e.g., positive and negative, that differ in voltage and current ratings. Two conventional topologies utilized in DC power supplies are a linear design topology and a switching design topology.
In the linear design topology, a low-frequency, e.g., 60 Hertz, transformer is used to provide electrical isolation between the input and output of the power supply with a transistor acting as an adjustable resistor. While the linear power supply employs a simple design and introduces a moderate electromagnetic interference (EMI) with other equipment employed therewith, the topology endures several limitations. First, low-frequency transformers are relatively large and, as a result, the dimensions of the linear power supply are constrained to accommodate a large low-frequency transformer. Due to the size limitations, the linear power supply is not preferable, especially in environments where the components are being downsized. Additionally, the transistor, acting as an adjustable resistor, operates within its active region, thereby resulting in a significant amount of power loss. Typically, the overall efficiencies of the linear power supplies are between 30% to 60%.
In contrast to the linear power supplies, the transformation of the DC voltage in switching power supplies is accomplished using DC/DC converters. The DC/DC converters usually employ solid-state devices, e.g., transistors, as switching devices that are completely on or completely off. Since the devices do not operate in the active region, power dissipation therethrough is significantly reduced, resulting in a higher efficiency converter; typically 70% to 90% efficient. Additionally, since switching power supplies employ a high frequency isolation transformer, the size and weight of the switching power supplies may be significantly reduced.
A switching, or switch-mode, power converter generally includes an inductor, or transformer, coupled to an input power source and a switching transistor. When the switching transistor is turned on, energy is supplied to the inductor or transformer from the input power source. When the switching transistor is off, the output stage, comprising a rectifying diode and an output capacitor, receives energy from the inductor and the input voltage source. The operation of the switching transistor is controlled such that the converter output is well-regulated.
Current approaches to providing an efficient, high power factor power converter with an isolated low DC output voltage includes using a buck-boost converter and its relatives (flyback, Cuk and SEPIC) that are known to draw theoretically perfect sine wave current when the converters are operated in a discontinuous conduction mode (DCM). In cost-sensitive applications, however, the above mentioned power converters are less attractive than a boost converter due to the poorer exploitation of the power switch and/or the additional inductor and capacitor. To produce an isolated low output voltage, a boost converter generally employs a nonisolated Boost stage that generates an intermediate high voltage level, e.g., 400 VDC. A second "chopper" stage then converts this intermediate voltage to the required low output level. The use of two power stages, i.e., boost and chopper stages, however, also increases the cost and lowers the overall efficiency of the power converter.
Accordingly, what is needed in the art is an improved power converter with an isolated low voltage output that mitigates the above mentioned limitations. In particular, there is a need in the art for a more efficient, high power factor flyback power converter that is operable in a critically continuous conduction mode.